The present invention relates to a magnetic recording medium and a magnetic recording/reproducing apparatus, particularly to a perpendicular magnetic recording medium having high recording density and a method for manufacturing the same, as well as a magnetic recording/reproducing apparatus that uses the same.
In recent years, the areal recording density of respective magnetic disk drives is being expanded by 100% annually. As the areal recording density is increased such way, however, a problem, so-called thermal decay of magnetization, has come to appear. This is why the conventional longitudinal magnetic recording is considered to be difficult to achieve the areal recording density over 7.75 gigabits.
On the other hand, unlike the conventional longitudinal magnetic recording, the perpendicular recording is characteristic in that the demagnetizing field that works between adjacent bits decreases in proportion to an increase of the linear recording density, whereby the recorded magnetization is stabilized. In addition, because a soft magnetic underlayer having high magnetic permeability is provided under the subject perpendicular recording layer to obtain a strong recording magnetic field, the coercivity of the perpendicular recording layer can be increased. Consequently, the perpendicular recording is now under examination as a recording method expected to overstep the limit of the thermal fluctuation of the conventional longitudinal recording.
One of the effective methods for realizing such high density recording with use of the perpendicular recording method is combining a double-layer perpendicular recording medium consisting of a soft magnetic underlayer and a perpendicular recording layer with a single pole type head. However, the double-layer perpendicular recording medium has been confronted with a problem; the medium includes a soft magnetic underlayer having high saturation flux density (Bs) and therefore, the following three points (1) to (3) are required to be improved to solve the problem. (1) The leakage magnetic flux from the magnetic domain wall of the soft magnetic layer is observed as spike noise. (2) The magnetic domain wall of the soft magnetic underlayer moves, whereby decay of magnetization occurs in the recorded magnetization. (3) Stray fields in the apparatus are concentrated at the recording head, whereby decay of magnetization occurs in the recorded magnetization just under the recording head.
Furthermore, because the soft magnetic underlayer is as thick as several tens of nanometers to several hundreds of nanometers, the surface smoothness of the underlayer is lost and this might affect the forming of the perpendicular recording layer and the fly ability of the recording head adversely.
One of the methods proposed for solving such a problem is, as disclosed in the official gazettes of JP-A Nos. 129946/1995 and 191217/1999, to provide a hard magnetic pinning layer between the soft magnetic layer and the substrate and orient the magnetization of the underlayer in one direction. The official gazette of JP-A No. 103553/1994 also proposes a method for suppressing the domain wall motion of the soft magnetic underlayer through exchange coupling with the anti-ferromagnetic in which magnetic spinning is oriented in one direction. In addition, the official gazette of JP-A No. 155321/2001 discloses another method, which reverses the orientation of the magnetization of the soft magnetic layer by forming the soft magnetic layer with two or more layers separated by a nonmagnetic layer respectively.